The invention relates to a circuit capable of extracting data from a signal which has been modulated by frequency shift keying (FSK). In such a system a sine wave carrier is modulated by altering its frequency. For digital data transmission the carrier has a frequency for each of the logic states employed. For example, in the well known Bell 103 standard for binary data, a space (logic zero) is represented by a 2025 Hz signal. The mark (logic one) is represented by a 2225 Hz signal. Other standards are available.
While the invention is useful in any FSK system it is of particular interest in integrated circuits. Efficient means of modulating and demodulating data have increasingly become the subject of investigation as integrated circuit technology has made digital data transfer a cost effective method of communication. Integrated circuit technology has recently made feasible the integration of complete digital modulation and demodulation systems on a single silicon chip. In so doing, it has been found that many previous architectures of FSK demodulators are unsuitable for integration, as the components required are not provided by current, cost effective integrated circuit fabrication techniques. On the other hand, previous circuit construction technologies did not provide some of the performance available in integrated circuit technologies. In integrated circuits area considerations are important because the smaller chip sizes provide economies in terms of increased device yield and lower semiconductor requirements. Since the invention results in substantially smaller capacitor values in a signal demodulator and since IC chip area is proportional to capacitor size, a significant area economy is available.